The present invention relates to an image forming apparatus which may be applied to a printer, a copier, a facsimile receiving apparatus, or the like, and more particularly to a technique of correcting gamma characteristics of an image forming apparatus.
The characteristics of an image forming apparatus vary in accordance with the environment including temperature, humidity and aged variation. Particularly, environmental variation and aged variation of gamma characteristics of an electrophotography process (i.e., relationships between a density value indicated by input image information and the corresponding density value of an output image) become problematic.
In order to solve the problem, Japanese Patent Publication No. 8-69210A discloses an image reproducing apparatus in which sample images that are called patch images and have different densities are printed as tests, and the densities of the RGB colors of each of the patch images that are formed on a sheet or a photosensitive member are measured optically or electrically. On the basis of results of the measurements, an appropriate one is selected from several kinds of gamma curves which are previously stored in a ROM for each of the RGB colors. The selected RGB gamma curves are used in gamma correction for the density level (tone level) of each of the RGB colors, in an image process preceding to an electrophotography process.
The related apparatus uses gamma curves which are selected from the several previously prepared gamma curves, and hence gamma characteristics of the image forming apparatus can be made close to ideal characteristics. However, it is impossible to accurately match gamma characteristics with the ideal ones because characteristics of an actual electrophotography process do not change in a manner strictly coincident with the previously prepared gamma curves. Furthermore, plural gamma curves must be previously programmed in the ROM. Therefore, the ROM is required to have a large capacity, so that the production cost of the apparatus is increased.
In the related apparatus, gamma correction is applied on the input RGB tone levels. Usually, final printing on a sheet is performed by using coloring agents (typically, toners) of CMYK. Therefore, the RGB tone levels which have undergone gamma correction must be converted into the CMYK tone levels by performing color conversion. When color conversion is performed, however, there arises a problem in that the accuracy of the gamma correction is lowered.